Why Isn't Terminal Potential Difference Equal to EMF When I=0?

[lha08]

Homework Statement

I'm just wondering for a terminal potential difference in a battery, I know the formula is V= E-Ir where E=electromotive force and r is the internal resistance. But in my textbook, it says that when I=0, that doesn't mean that V is the same thing as E...it says 'the condition I=0 represents a balance between two opposing tendencies: The charges tend to minimize their electrostatic potential energy and the source of emf tends to separate them, and thereby to minimize some other form of energy, such as chemical bond energy.' I have trouble picturing what exactly they're trying to imply..could someone clear things up? Like I don't know what 'charges' they're talking about or how the emf is separating them if there's no current flow...

Homework Equations

The Attempt at a Solution

 

[jbsteele]

When I=0, this means that the current in a circuit is zero, or in other words, no charge is flowing through the circuit. This means that the charges in the battery are not moving, because there is no current flow. In this situation, the charges tend to minimize their electrostatic potential energy, meaning that the charges will try to become as close together as possible. On the other hand, the source of EMF (electromotive force) is trying to separate them, because it is trying to minimize some other form of energy, such as chemical bond energy. This is why the terminal potential difference (V) is not equal to the EMF (E) when I=0.